.3

SCHEDULE

Tuesday 11th September 2012 Place VINCCI GRANADA HOTEL

17:00 - 20:00 Registration

Wednesday 12th September 2012 Place VINCCI GRANADA HOTEL

Starting at 08:00

Registration

Place RECTORATE OF THE UNIVERSITY OF GRANADA 10:00 - 10:45 OPENING SESSION 10:45 - 11:30 KEYNOTE 1: PROF. KANJI ONO

Place VINCCI GRANADA HOTEL 12:00 - 13:20 Lunch Break

Room Granada Hall Mediterraneo Hall Session S1

AE in Different Applications S2

AE in Advanced Composites

13:30 - 13:50 A. POLLOCK M. PLCKL

13:50 - 14:10 M. ENOKI D. CRIVELLI

14:10 - 14:30 A. ANASTASOPOULOS M. NOWAK

14:30 - 14:50 M. GOMEZ

Session S3 AE in Aerospace Structures and

Composites

S4 AE in Wood and Biological

Applications

15:00 - 15:20 A. PROUST S. ROSNER

15:20 - 15:40 L. ADAM P. MAZAL

15:40 - 16:00 M. CHLADA C. GROSSE

16:00 16:30 Coffee Break Session S5

AE Signal Processing and Source Location (1)

S6 AE in Pipelines, Vessels, Tanks and

Leaks (1)

16:30 16:50 R. PIOTRKOWSKI M. SHEHADEH

16:50 17:10 E. POMPONI T. THENIKL

17:10 17:30 S. MOHD I. RAZUVAEV

17:30 17:50 D. TERENTYEV C. HERVE

17:50 18:10 P. MCCRORY X. QIN

4

Wednesday 12th September 2012 DINNER 20:30

PALACE CARMEN DE LOS MRTIRES (Departing from Vincci Granada Hotel by bus at 20:00)

Thursday 13th September 2012

Place VINCCI GRANADA HOTEL Room Granada Hall Mediterraneo Hall

Session S7 AE Sensors and Transducers

S8 AE in Geological Applications and

Rocks

09:00 - 09:20 S. DIA S. STANCHITS

09:20 - 09:40 H. HATANO G. MANTHEI

09:40 - 10:00 A. J. BRUNNER C. HUCAILUK

10:00 - 10:20 P. BENES M. POSCOLIERI

10:20 - 10:50 Coffee Break 10:50 - 11:30 Granada Hall Room: KEYNOTE 2: PROF. TOMOKI

SHIOTANI Session S9

AE in Industrial and Transport S10

AE in Electrochemical Applications and Materials Characterization

11:30 - 11:50 D. KUSIC P. X. THIVEL

11:50 - 12:10 R. L. REUBEN A. ETIEMBLE

12:10 - 12:30 P. TSCHELIESNIG M. NEYRA

12:30 - 12:50 I. BARAN

12:50 - 14:00 Lunch Break Session S11

AE for Materials Characterization S12

AE in Signal Processing and Source Location (2)

14:00 - 14:20 F. RAVNIK V. BARAT

14:20 - 14:40 K. YOSHIDA F. RAUSCHER

14:40 - 15:00 A. VINOGRADOV K. ITO

15:00 - 15:20 Y. MIZUTANI V. KUS

15:20 - 15:40 S. WAKAYAMA 15:40 - 16:10 Coffee Break 16:10 - 18:00 Granada Hall Room: EWGAE BUSINESS MEETING

Thursday 13th September 2012 COCKTAIL

20:30 CARMEN DE LOS CHAPITELES

(Going on your own. Organized groups will depart from the Vincci Hotel at different times)

5

Friday 14th September 2012 Place VINCCI GRANADA HOTEL Room Granada Hall Mediterraneo Hall

Session S13 AE in Advanced Composites and Lamb

Waves

S14 AE in Pipelines, Vessels, Tanks and

Leaks (2)

09:00 - 09:20 M. HAMSTAD E. NEFEDYEV

09:20 - 09:40 M. SAUSE Z. FAN

09:40 - 10:00 K. ONO I. BARAN

10:00 - 10:20 M. J. PEACOCK

10:20 - 10:50 Coffee Break 10:50 - 11:30 Granada Hall Room: KEYNOTE 3: PROF. ZDENEK

PREVOROVSKY Session S15

AE in Concrete and Construction Materials

S16 AE in Large Structures

11:30 - 11:50 E. VERSTRYNGE A. SAGAIDAK

11:50 - 12:10 E. PROVERBIO B. MURAVIN

12:10 - 12:30 A. POLLOCK S. BUDANO

12:30 - 13:40 Lunch Break Session S17

AE Instrumentation and Courses S18

General Aspects in AE

13:40 - 14:00 S. ELIZAROV W. BEN KHALIFA

14:00 - 14:20 H. VALLEN A. LHMERY

14:20 - 14:40 V. GENIS M. BEN TAHAR

14:40 - 15:00 G. POR T. PETERSEN

Friday 14th September 2012 DINNER 20:30

RESTAURANT LA CHUMBERA (Departing from Vincci Granada Hotel by bus at 19:45)

Saturday 15th September 2012 ALHAMBRA Optional guided visit to the Alhambra

(Departing from Vincci Granada Hotel by bus at 08:30)

COCKTAIL AND CLOSING SESSION

Alhambra Palace Hotel

6

CONTENTS

PAPERS from ORAL PRESENTATIONS Pages 1 - 44

KEYNOTE 1: Prof. KANJI ONO

Research and Applications of AE on Advanced Composites

SESSION 1: AE in Different Applications

Chairs: G. Shen and V. Svoboda Room: Granada Hall

45 - 58 AE and Other Observations during Cyclic Testing of A572 Steel Laboratory Specimens Adrian A. POLLOCK, Jianguo (Peter) YU, Paul ZIEHL USA

59 - 66 Quantitative Acoustic Emission Measurement of Laser Peening Manabu ENOKI, Kazutaka KOBAYASHI, Tomoki TAKATA, Akinori MATSUI, Yuji KOBAYASHI, Japan

67 - 80 Health Monitoring of a NEG-MICON 750kW Wind Turbine Blades with Acoustic Emission Dimitrios PAPASALOUROS, Nikos TSOPELAS, Ioannis LADIS, Dimitrios KOUROUSSIS, Athanasios ANASTASOPOULOS, Denja LEKOU, Fragiskos MOUZAKI, Greece

81 - 93 Correlation Between Acoustic Emission and Torque in Drilling Martin GOMEZ, Jos RUZZANTE, Argentina

SESSION 2: AE in Advanced Composites Chairs: M. Hamstad and I. Baran

Room: Mediterraneo Hall 94 - 105 Failure Analysis of NOL-Ring Specimens by

Acoustic Emission Marina PLCKL, Markus G.R. SAUSE, Joachim SCHARRINGHAUSEN, Siegfried HORN, Germany

106 - 118 An Investigation of the Static Damage Mechanisms of Pultruded Glass Fiber Reinforced Polymers with Artificial Neural Networks Davide CRIVELLI, Mario GUAGLIANO, Alberto MONICI, Italy

7

119 - 125 Evaluation of the Destruction Process of Metal-Composite Cylinders with Use of Acoustic Emission Method Jerzy SCHMIDT, Marek NOWAK, Ireneusz BARAN, Poland

SESSION 3: AE in Aerospace Structures and Composites Chairs: M. Chlada and Y. Mizutani

Room: Granada Hall

126 - 137 Non Destructive Testing of GAIA Frame by Means of Acoustic Emission Monitoring During Launch Simulation Tests Franck LEVALLOIS, Andr SOBECZKO, Alain PROUST, Jean-Claude LENAIN, France

138 - 149 Benefits of Acoustic Emission for the Testing of Aerospace Composite Assemblies Louis ADAM, Elodie BONHOMME, Clment CHIROL, Alain PROUST, France

150 - 157 Remote AE Monitoring of Fatigue Crack Growth in Complex Aircraft Structures Milan CHLADA, Zdenek PREVOROVSKY, Czech Republic

SESSION 4: AE in Wood and Biological Applications Chairs: M. Gmez and S. Rosner

Room: Mediterraneo Hall

158 - 166 Acoustic Emission Related to Stomatal Closure of Four Deciduous Broad-Leaved Woody Species Sabine ROSNER, Austria

167 - 174 Correlation of Sap Flow Changes in Trees with Signal of Acoustic Emission during Field Measurements Pavel MAZAL, Michal CERNY, Frantisek VLASIC, Libor NOHAL, Czech Republic

175 - 184 Investigation of Failure Processes in the Human Femur Using Signal-Based Acoustic Emission Techniques Florian STARK, Fabian MALM, Eduardo GRANDE GARCIA, Peter FHR, Rainer H. BURGKART, Franziska REIDL, Christian GROSSE, Germany

8

SESSION 5: AE Signal Processing and Source Location (1) Chairs: A. Vinogradov and V. Barat

Room: Granada Hall

185 - 197 AE Signal Processing in Dynamical Tests of Reinforced Concrete Structures Miguel Eduardo ZITTO, Mariano SCARAMAL, Francisco A. SAGASTA , Rosa PIOTRKOWSKI, Antolino GALLEGO, Enrique CASTRO, Argentina

198 - 205 Real-time Acoustic Emission Classification: New evolutionary Approach Eraldo POMPONI, Alexei VINOGRADOV, Russia

206 - 216 Acoustic Emission Source Location in Steel Structures Using a Wavelet Transform Analysis and Modal Location Theory Shukri MOHD, Karen HOLFORD, Rhys PULLIN, United Kingdom

217 - 221 Use of Lamb Wave Dispersion Curves Extraction from AE Signal Spectrogram for Determination of Distance to AE Source Denis TERENTYEV, Yuri BORODIN, Russia

222 - 230 Comparison of Time of Arrival and Delta T Source Location for Acoustic Emission in GLARE John Patrick MCCRORY, Rhys PULLIN, Matthew Robert PEARSON, Mark Jonathan EATON, Carol A FEATHERSTON, Karen M HOLFORD, United Kingdom

SESSION 6: AE in Pipelines, Vessels, Tanks and Leaks (1) Chairs: M. Peacock and R. Reuben

Room: Mediterraneo Hall

231 - 242 Monitoring Erosion-Corrosion in Carbon Steel Elbow Using Acoustic Emission Technique Mohamed SHEHADEH, Ibrahim HASSAN, Hany MOURAD, Hassan EL-GAMAL, Egypt

243 - 249 New Intrinsically Safe Acoustic Emission Sensor System for Use on Tank Ships Thomas THENIKL, Hartmut VALLEN, Dirk ALJETS, Germany

250 - 255 Analytical Verification of the Active AE Sources in the Integrated SHM Systems Igor RAZUVAEV, Evgeny A. SUCHKOV, Russia

9

256 - 268 Acoustic Emission Monitoring of High Temperature Crack Propagation in AISI 304L and P265GH Catherine HERVE, Fethi DAHMENE, Abdelouaedh LAKSIMI, Lionel JAUBERT, Said HARIRI, Mohammed CHERFAOUI, Anne MOUFTIEZ, France

269 - 279 Theoretical Analysis and Experimental Study of Gas Pipeline Leak Acoustic Emission Signal Transmit Speed Gongtian SHEN, Xianyong QIN, Renyang HE, Changzheng XILI, China

SESSION 7: AE Sensors and Transducers

Chairs: P. Benes and H. Vallen Room: Granada Hall

280 - 287 Primary Calibration of Acoustic Emission

Sensors by the Method of Reciprocity, Theoretical and Experimental Considerations Seydou DIA, Thomas MONNIER, Nathalie GODIN, Fan ZHANG, France

288 - 295 Absolute Calibration of Acoustic Emission Transducers as per CEN ISO/TR 13115 in Disuse of Mechanical Sound Sources or Reference Transducers Hajime HATANO, Japan

296 - 306 Acoustic Emission Monitoring of Tensile Tests on Packaged Piezoceramic Wafer Transducers Andreas J. BRUNNER, Florian BACHMANN, Switzerland

307 - 315 Comparison of Methods for Absolute Calibration of Wideband AE sensors Petr BENES, Jiri FIALKA, Czech Republic

SESSION 8: AE in Geological Applications and Rocks Chairs: E. Proverbio and G. Manthei

Room: Mediterraneo Hall

316 - 329 Acoustic Emission and Ultrasonic Transmission Monitoring of Hydraulic Fracture Initiation and Propagation in Rock Samples Sergey STANCHITS, Aniket SURDI, Roberto SUAREZ-RIVERA, USA

10

330 340 Application of the Cluster Method on Acoustic Emission Events in Salt Rock Specimen Gerd MANTHEI, Jrgen EISENBLTTER, Germany

341 353 Acoustic Emission on the Peteroa Volcano (2009-2011), Argentina Maria Isabel LOPEZ PUMAREGA, Cynthia HUCAILUK, Giovanni P. GREGORI, Grabiele PAPARO, Jose Evaristo RUZZANTE, Nicolas NUEZ, Giuliano VENTRICE, Claudio RA-FANELLI Argentina

354 363 RQA Applied to Acoustic Emissions Time Series to Pinpoint Feasible Precursors of Catastrophic Geological Events Maurizio POSCOLIERI, Claudio RAFANELLI, Giovanna ZIMATORE, Italy

364 -384

KEYNOTE 2: Prof. TOMOKI SHIOTANI

Recent advances of AE technology for damage assessment of infrastructures

SESSION 9: AE in Industrial and Transport Chairs: A. Anastasopoulos and A. Brunner

Room: Granada Hall

385 -393 Use of AE Method for Detecting Quality of Injection Mold Engraving Inserts Dragan KUSIC, Tomaz KEK, Janez Marko SLABE, Rajko SVECKO, Janez GRUM, Slovenia

394 406 Acoustic Emission in Diesel Engines A Time Series Model for the Injection Process Wael ABDOU, Nikolaos BALODIMOS, Bob REUBEN, United Kingdom

407 413 Acoustic Emission (AE) Monitoring on Transport Products Peter TSCHELIESNIG, Austria

414 426 Acoustic Emission Monitoring of Structural Elements a Ship for Detection of Fatigue and Corrosion Damages Ireneusz BARAN, Marek NOWAK, Andreas JAGENBREIN, Henryk BUGLACKI, Poland

SESSION 10: AE in Electrochemical Applications and Materials Characterization

Chairs: S. Wakayama and A. Laksimi

11

Room: Mediterraneo Hall 427 436 Acoustic Methods as a Tool for Management of

Electrochemical Process of Energy Nina KIRCHEVA, Sylvain TANT, Benoit LEGROS, Sylvie GENIES, Sebastien ROSINI, Pierre Xavier THIVEL, France

437 - 445 On the Use of the Acoustic Emission Technique for in-situ Monitoring of the Pulverization of Battery Electrodes Aurlien ETIEMBLE, Hassane IDRISSI, Lionel ROU, France

446 - 455 Magneto Acoustic Emission and Barkhausen Noise in A 508 Class II Steel Miriam Rocio NEYRA ASTUDILLO, Nicols NUEZ, Isabel LOPEZ PUMAREGA, Daro N. TORRES, Jos RUZZANTE, Argentina

SESSION 11: AE for Material Characterization Chairs: A. Pollock and E. Martnez

Room: Granada Hall

456 468 Assessment of Sound Emission Phenomena at a Boundary Layer During Steel Quenching Franc RAVNIK, Janez GRUM, Slovenia

469 475 Evaluation of AE Sources during Tensile Deformation of Al-Mg-Si Alloys with Different Heat Treatment Kenichi YOSHIDA, Yuichi TOKUYAMA, Yoshihiro YASUHARA, Hideo NISHINO, Japan

476 487 Mechanisms of Plastic Deformation and Acoustic Emission in ZK60 Mg Alloy Alexei VINOGRADOV, Alexei DANYUK, Eraldo POMPONI, Russia

488 - 497 Effect of Plastic Strain on Low-Cycle Fatigue Life and Fatigue Damage Detection Yoshihiro MIZUTANI, Makoto OHTA, Akira TODOROKI Japan

498 - 505 AE Studies on the Durability of Flexible Dye-Sensitized Solar Cells Shuichi WAKAYAMA, Nobuyuki TAKAGI, Takenobu SAKAI, Masashi IKEGAMI, Tsutomu MIYASAKA, Japan

SESSION 12: AE in Signal Processing and Source Location (2) Chairs: R. Piotrkowski and F. Rauscher

Room: Mediterraneo Hall

12

506 - 515 Application of ICI Principle for the First-Arrival Time Correction Vera BARAT, Sergey ELIZAROV, Irina KOVALCHUK, Evgeniy BOLOKHOV, Russia

516 - 529 Using X22-Correlation for Subclustering and Relative Location of Acoustic Amission Signals Franz RAUSCHER, Austria

530 - 535 AE Event Detection under Noisy Environment with Continuous Waveform Recording and Multiple Thresholds Kaita ITO, Hitoshi KURIKI, Seiji KURODA, Manabu ENOKI, Japan

536 - 547 Acoustic Emission Defects Localized by Means of Geodetic Iterative Procedure - Algorithms, Tests, AE Experiment Vaclav KUS, Michal ZAVESKY, Zdenek PREVOROVSKY Czech Republic

SESSION 13: AE in Advanced Materials and Lamb Waves Chairs: Z. Prevorovsky and A. Proust

Room: Granada Hall

548 561 A Waveform-Based Study of AE Wave Propagation by Use of Eight Sensors on a Composite Pressure Vessel Marvin HAMSTAD, USA

562 - 572 Influence of Internal Discontinuities on Ultrasonic Signal Propagation in Carbon Fiber Reinforced Plastics Markus SAUSE, Siegfried HORN, Germany

573 - 580 An Improved Acousto-Ultrasonic Scheme with Lamb Wave Mode Separationand Damping Factor in CFRP Plates Antolino GALLEGO, Kanji ONO, Spain

SESSION 14: AE in Pipeline, Vessels, Tanks and Leaks (2) Chairs: C. Herve and E. Romero

Room: Mediterraneo Hall

13

581 - 589 Remote Sensing of Leaks Parameters by Acoustic Emission Method Evgeny NEFEDYEV, Russia

590 - 598 Qualification of the Acoustic Emission Method for Leak Detection in the Main Primary Loop of the Pressurized Water Reactors During its Hydraulic Pressure Test Yann FORESTIER, Zhang FAN, Laurent TRUCHETTI, France

599 - 610 Analysis of Corrosion Processes and Leaks in Aboveground Storage Tanks in Aspect of AE Monitoring system Ireneusz BARAN, Gerald LACKNER, Poland

611 - 625 Improved Noise Discrimination for Ammonia Tank Testing Martin J PEACOCK, United Kingdom

626 - 640

KEYNOTE 3: Prof. ZDENEK PREVOROVSKY

Progressive Approaches to Localization and Identification of AE Sources

SESSION 15: AE in Concrete and Construction Materials

Chairs: T. Shiotani and C. Grosse Room: Granada Hall

641 - 654 Initial Results on Acoustic Amission Detection

in Masonry with Optical Fibre Sensors Els VERSTRYNGE, Martine WEVERS, Belgium

655 665 Multivariate and Artificial Neural Network Analyses of AE Data during Long Time Corrosion Monitoring of a Post-Tensioned Concrete Beam Edoardo PROVERBIO, Luigi CALABRESE, Giuseppe CAMPANELLA, Italy

666 - 678 Low-Level AE During Long-Term Monitoring of Concrete Adrian A. POLLOCK, USA SESSION 16: AE in Large Structures

Chairs: P. Tscheliesnig and V. Svoboda Room: Mediterraneo Hall

14

679 - 686 Experience of Acoustic Emission Method Application for Estimation of Building Structures Condition Alexander SAGAIDAK, Sergey Elizarov, Russia

687 698 Guide for Acoustic Emission Examination of Reinforced Concrete Bridges Gregory MURAVIN, Boris MURAVIN, Israel

699 710 Damages Monitoring for Oil&Gas Pipeline using AE Technique Sergio BUDANO, Antonio LUCCI, Roberto PIANCALDINI, Luca PRANDI, GIUSEPPE GIUNTA, Italy

SESSION 17: AE Instrumentation and Courses Chairs: M. Enoki and I. Lpez-Pumarega

Room: Granada Hall

711 - 717 The Multipurpose Portable NDT Device UNISCOPE Sergey Vladimirovich ELIZAROV, Alexander L. ALYAKRITSKIY, Vassily G. KOLTSOV, Vera A. BARAT, Pavel N. TROFIMOV, Russia

718 728 New Intrinsically Safe Acoustic Emission Sensor System for Use on Tank Ships Hartmut VALLEN, Thomas THENIKL, Germany

729 - 741 Nondestructive Evaluation Courses for Undergraduate Engineering and Engineering Technology Students Vladimir GENIS, Adrian POLLOCK, USA

742 - 752 Developing an Acoustic Emission Measuring System Based on Modular High Speed Data Acquisition Devices Gabor POR, Laszlo DOSZPOD, Tibor DOBJAN Hungary SESSION 18: General aspects in AE

Chairs: M. Sause and K. Yoshida Room: Mediterraneo Hall

753 - 765 Modeling of the Far-Field Acoustic Emission

from a Crack Under Stress Warida BEN KHALIFA, Karim JEZZINE, Sbastien GRONDEL, Alain LHMERY, Sylvain CHATILLON, France

15

766 - 775 Acoustic Emission during Crack Growth by a Hybrid Semi-Analytical/BEM Model Mabrouk BEN TAHAR, Gaetan HELLO, Nacera BEDRICI, France

776 - 783

Analysis of Microcrack Size and Acoustic Emission Amplitude Distributions Tatiana B. PETERSEN1, Lyudmila R. BOTVINA2, Victor V. SHEMYAKIN, Russia

PAPERS from POSTER PRESENTATIONS

784 795 AE Study of Damage Evolution in Pressure Vessels under Cyclic Loading Catherine HERVE, France 796 - 803 Experience on "AFIAP" Guide to Good Practice for Acoustic Emission Testing of Pressure Equipment Catherine HERVE, France 804 811 Failure Mechanisms of Carbon-Fiber-Reinforced Polymer Materials Characterized by Acoustic Emission Techniques Christian GROSSE, Germany 812 818 An Interesting AE Proficiency Test Janos GEREB, Hungary 819 825 Experimental Study on Feature Selection Using Artificial AE Sources

Gabriel CSEFALVAY, Czech Republic 826 838 Maintenance Scheduling and Improve Bridge Load Management Sp SANTOSPIRITO, United Kingdom 839 845 Mechanical Characterization of the Damage Process in a Structural Adhesive Joint by Acoustic Emission Francisco Javier JUANES, Spain 846 854 3D Model Based on Regular Cubic Cell Mesh for AE Sources Location on Complex Industrial Structures Alexey B. SAMOKHVALOV, Victor V. SHEMYAKIN, Russia

2012

AUTHORS INDEX Wael ABDOU, 394 Louis ADAM, 138 Dirk ALJETS, 243 Alexander L. ALYAKRITSKIY, 711 Athanasios ANASTASOPOULOS, 67 Florian BACHMANN, 296 Nikolaos BALODIMOS, 394 Ireneusz BARAN, 119, 414, 599 Vera A. BARAT, 506, 711 Nacera BEDRICI, 776 Warida BEN KHALIFA, 753 Mabrouk BEN TAHAR, 776 Petr BENES, 307 Evgeniy BOLOKHOV, 506 Elodie BONHOMME, 138 Yuri BORODIN, 217 Andreas J. BRUNNER, 296 Sergio BUDANO, 699 Henryk BUGLACKI 414 Rainer H. BURGKART, 175 Luigi CALABRESE, 655 Giuseppe CAMPANELLA 655 Enrique CASTRO, 185 Michal CERNY, 167 Sylvain CHATILLON, 753 Mohammed CHERFAOUI, 256, 784, 796 Clment CHIROL, 138 Milan CHLADA, 150, 626 Davide CRIVELLI, 106 Gabriel CSEFALVAY, 819 Fethi DAHMENE, 256 Alexei DANYUK, 476 C. DI GIULIO, 796 Seydou DIA, 280 Tibor DOBJAN 742 Laszlo DOSZPOD, 742 Mark Jonathan EATON, 222 Jrgen EISENBLTTER, 330 Hassan EL-GAMAL, 231 Sergey Vladimirovich ELIZAROV, 506, 679, 711 C. ENNACEUR, 784 Manabu ENOKI, 59, 530 Aurlien ETIEMBLE, 437 Zhang FAN, 590 Zuzana FAROVA, 626 Carol A. FEATHERSTON, 222 Jiri FIALKA, 307 Peter FHR, 175 Yann FORESTIER, 590 Ferenc FCSK 812 Antolino GALLEGO, 1, 185, 573, 839 Sylvie GENIES, 427 Vladimir GENIS, 729 Janos GEREB, 812 Giuseppe GIUNTA, 699 Nathalie GODIN, 280

Martin GOMEZ, 81 Maria Luz GONZLEZ, 839 Eduardo GRANDE GARCIA, 175 Giovanni P. GREGORI, 341 Sbastien GRONDEL, 753 Christian GROSSE 175, 804 Janez GRUM, 385, 456 Mario GUAGLIANO, 106 Marvin HAMSTAD, 548 Said HARIRI, 256 Ibrahim HASSAN, 231 Hajime HATANO, 288 Renyang HE, 269 Gaetan HELLO, 776 C. HERVE 256, 784, 796 Karen M. HOLFORD, 206, 222 Siegfried HORN, 94, 562 Cynthia HUCAILUK, 341 Hassane IDRISSI, 437 Masashi IKEGAMI, 498 Kaita ITO, 530 Andreas JAGENBREIN, 414 Peter JAHNKE, 804 Lionel JAUBERT, 256 Karim JEZZINE, 753 Francisco Javier JUANES, 839 Tomaz KEK, 385 Nina KIRCHEVA, 427 Kazutaka KOBAYASHI, 59 Yuji KOBAYASHI, 59 Vassily G. KOLTSOV, 711 Dimitrios KOUROUSSIS, 67 Irina KOVALCHUK, 506 Josef KROFTA, 626 Hitoshi KURIKI, 530 Seiji KURODA, 530 Vaclav KUS 536, 626 Dragan KUSIC, 385 Gerald LACKNER, 599 Ioannis LADIS, 67 Abdelouaedh LAKSIMI, 256, 784 Benoit LEGROS, 427 Denja LEKOU, 67 Jean-Claude LENAIN, 126 Franck LEVALLOIS, 126 Alain LHMERY, 753 Maria Isabel LOPEZ PUMAREGA, 341, 446 Antonio LUCCI, 699 Fabian MALM, 175 Gerd MANTHEI, 330 Akinori MATSUI, 59 Pavel MAZAL, 167 John Patrick McCRORY, 222 Heinrich MEYER, 804 Jos Carlos MGUEZ, 839 Tsutomu MIYASAKA, 498

16

2012 Yoshihiro MIZUTANI, 488 Shukri MOHD, 206 Alberto MONICI, 106 Thomas MONNIER, 280 Anne MOUFTIEZ, 256 Hany MOURAD, 231 Fragiskos MOUZAKI, 67 Boris MURAVIN, 687 Gregory MURAVIN, 687 Evgeny NEFEDYEV 581 Miriam Rocio NEYRA ASTUDILLO, 446 Hideo NISHINO, 469 Libor NOHAL, 167 Marek NOWAK, 119, 414 Nicols NUEZ, 341, 446 Makoto OHTA, 488 Kanji ONO, 1, 573 Grabiele PAPARO, 341 Dimitrios PAPASALOUROS, 67 Martin J. PEACOCK, 611 Matthew Robert PEARSON, 222 Roberto PIANCALDINI, 699 Rosa PIOTRKOWSKI, 185 Marina PLCKL, 94 Adrian A. POLLOCK, 45, 666, 729 Eraldo POMPONI, 198, 476 Gabor POR, 742 Maurizio POSCOLIERI, 354 Luca PRANDI, 699 Zdenek PREVOROVSKY, 150, 536, 626 Alain PROUST, 126, 138 Edoardo PROVERBIO, 655 Rhys PULLIN, 206, 222 Xianyong QIN, 269 Martin RADLMEIER, 804 Claudio RAFANELLI, 341, 354 Franz RAUSCHER, 516 Franc RAVNIK, 456 Igor RAZUVAEV, 250 Franziska REIDL, 175 Bob REUBEN, 394 Sebastien ROSINI, 427 Sabine ROSNER, 158 Lionel ROU, 437 Jos RUZZANTE, 81, 341, 446 Alexander SAGAIDAK, 679 Francisco A. SAGASTA , 185 Takenobu SAKAI, 498 Alexey B. SAMOKHVALOV, 846 S.P. SANTOSPIRITO, 826 Markus G.R. SAUSE, 94, 562 Mariano SCARAMAL, 185 Joachim SCHARRINGHAUSEN, 94 Jerzy SCHMIDT, 119 Mohamed SHEHADEH, 231 Victor V. SHEMYAKIN, 846 Gongtian SHEN, 269 Tomoki SHIOTANI 364 Janez Marko SLABE, 385

Kamil SYK 826 Andr SOBECZKO, 126 Sergey STANCHITS, 316 Florian STARK, 175 Roberto SUAREZ-RIVERA, 316 Evgeny A. SUCHKOV 250 Aniket SURDI, 316 Rajko SVECKO, 385 Nobuyuki TAKAGI, 498 Tomoki TAKATA, 59 Sylvain TANT, 427 Thomas THENIKL, 243, 718 Pierre Xavier THIVEL, 427 Akira TODOROKI 488 Yuichi TOKUYAMA, 469 Daro N. TORRES, 446 Denis TERENTYEV, 217 Pavel N. TROFIMOV, 711 Laurent TRUCHETTI 590 Peter TSCHELIESNIG, 407 Nikos TSOPELAS, 67 Angel VALEA, 839 Hartmut VALLEN, 243, 718 Giuliano VENTRICE, 341 Els VERSTRYNGE, 641 Alexei VINOGRADOV, 198, 476 Frantisek VLASIC, 167 Shuichi WAKAYAMA, 498 Robert WALKER, 826 Martine WEVERS, 641 Changzheng XILI 269 Yoshihiro YASUHARA, 469 Kenichi YOSHIDA, 469 Jianguo (Peter) YU, 45 Michal ZAVESKY, 536 Fan ZHANG, 280 Paul ZIEHL 45 Giovanna ZIMATORE, 354 Miguel Eduardo ZITTO, 185

17

Proceedings of the 30th European Conference on Acoustic Emission Testing

& 7th International Conference on Acoustic Emission

EWGAE 30/ ICAE 7 Granada, September 2012

Edited by

Prof. Antolino Gallego Prof. Kanji Ono

Venue

Vincci Granada Hotel Granada, SPAIN

September 12 - 15, 2012

Conference Arrangements University of Granada

Dr. Antolino Gallego, Conference Host

Organizer University of Granada

Supported by:

EWGAE, the European Working Group on Acoustic Emission AEWG, the Acoustic Emission Working Group

JCAE, the Japanese Committee on Acoustic Emission GLEA, the Latin American working group on Acoustic Emission

CCAE, the Chinese Committee on Acoustic Emission

Host UNIVERSITY OF GRANADA

Produced by NDT.net - The Open Access NDT Database, Germany

Copyright 2012 University of Granada, Printed in Spain and EWGAE

All rights reserved. No part of the contents of this book may be reproduced or transmitted in any form or by any means without written permission of the publisher or the author. Neither the publisher nor the producer is responsible for the use which might be made of the following information. Every effort has been made to ensure that these proceedings are correct. An online version of these proceedings is available at www.ndt.net

18

Distributed by 30 EWGAE / 7 ICAE Organizing Committee Department of Applied Physics ETS de Ingeniera de Edificacin University of Granada 18071 Granada, Spain www.ewgae.eu; www.2012.ewgae.es ewgae2012@ewgae.es

Purpose of Conference This Conference is intended to provide an international forum for delegates to present and discuss research and industrial applications of the science and technology of acoustic emission (AE). We wanted to explore both theoretical and practical aspects of acoustic emission associated with materials characterization, structural integrity monitoring, in-process monitoring, instrumentation development, as well as pertinent codes and standards and to set directions for future developments and applications in the field of acoustic emission. The 30th conference of the European working group on Acoustic Emission (EWGAE), held in Granada, Spain, in September 2012 has been organized by University of Granada. EWGAE-30 has been also supported by

AEWG, the Acoustic Emission Working Group, JCAE, the Japanese Committee on Acoustic Emission, GLEA, the Latin American working group on Acoustic Emission, and CCAE, the Chinese committee on Acoustic Emission.

Due to this overwhelming international support, EWGAE-30 has been combined with ICAE-7, the 7th International Conference on Acoustic Emission.

The European Working Group on Acoustic Emission The European Working Group on Acoustic Emission (EWGAE) was formed in 1972 by a number of people throughout Europe who were already engaged in acoustic emission research. At that time, Acoustic Emission research was performed in England by Adrian Pollock at Cambridge Consultants, by Don Birchon at the Admiralty Materials Laboratory, by Peter G. Bentley at Risley Engineering and at the Materials Laboratory of the U.K. Atomic Energy Authority, by Brian Harris at the University of Sussex; in Germany by Jrgen Eisenbltter and Peter Jax at Battelle Institut; in France by P. F. Dumousseau at CETIM, by Madame Nicole Chretien and E.G. Tomachevsky at Centre d'Etudes Nucleaires de Saclay, Commissariat a l'Energie Atomique; in Italy by M. Mirabile at Centro Sperimentale Metallurgico in Rome; in Denmark by Arved Nielsen at the Research Establishment Riso, Danish Atomic Energy Commission; in The Netherlands by J.C.F. DeKanter at the Technische Hogeschool Delft. (This list may not be complete.) The first European meeting on the subject was the "Institute of Physics Conference on Acoustic Emission" which was organized by Adrian Pollock and held in March 1972 at Imperial College in London. Fifteen papers from throughout Europe and USA were presented. The success of this conference established both the interest and the need for forming a working group. Consequently, Pollock and Birchon organized the European Stress Wave Emission Working Group, which held its first meeting in November 1972, at the

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Admiralty Materials Laboratory in England. During their second meeting at Battelle-Institut in September 1973, the group formally adopted their present name, European Working Group on Acoustic Emission. The original letterhead logo was designed by Patricia Preston, graphics designer at Cambridge Consultants. During the late 1970s and early 1980s, the Codes Subgroup of EWGAE published 5 codes of practice. In 1991, the work of this group was taken up by a new working group on AE (WG7) within TC138, the Technical Committee for NDT within CEN. This working group is comprised of national representatives who have been nominated by their respective National Standards Institutions. The first Convenor of WG-7 was Emilio Fontana of CISE, Italy. After his retirement in 2003, Peter Tscheliesnig of TV Austria was appointed and is currently serving as Secretary of the group. Since 2000, the following standards for AE, developed by TC138-WG7 have been published:

EN 1330-9: NDT - Terminology - Part 9: Terms used in AE testing EN 13544: NDT - AE - General principles EN 13477-1: NDT - AE - Equipment characterisation, Part 1: Equipment description EN 13477-2: NDT - AE - Equipment characterisation, Part 2: Verification of

operating characteristics EN 14584: NDT - AE - Examination of metallic pressure equipment during proof

testing - planar location of AE sources EN 15495: Examination of metallic pressure equipment during proof testing - Zone

location of AE sources.

Current work items deal with testing of fiber-reinforced polymers, AE testing for the detection of corrosion within metallic surrounding filled with liquid, and reviews of EN 13554 and 13477-2. These Standards have significantly contributed to the today's acceptance of the AE testing method in Europe. During the last 5 years, active members within the CEN TC138-WG7 were: P. Tscheliesnig (Austria), C. Herve and J.C. Lenain (France), J. Bohse and H. Vallen (Germany), A. Anastasopoulos (Greece), C. De Petris (Italy), E. Romero (Spain), H. Schoorlemmer (The Netherlands), L. Rogers and P. Cole (U.K.) Since 2000, the European Standard for Qualification and Certification of NDT Personnel, EN473, covers also the AE testing method (abbreviation: AT) and defines details for education and certification. During 2005, the British Institute of Non-Destructive testing has established an Acoustic Emission Working Group to develop the Acoustic Emission sections of their PCN scheme which provides an international programme for the certification of testing, inspection and condition monitoring personnel which satisfies the requirements of numerous European and international standards. Members of this group include K Holford, T Bradshaw, D Mba, M Forde, Trevor Holroyd, Winfield Stewart, Ian Taylor, John O'Brien, R Reuben, Stuart Courtenay and Martin Peacock. The following EWGAE conferences were held since 2000: Senlis/Paris, France (June 2000); Prague, Czech Republic (Sept. 2002); Berlin, Germany (Sept. 2004); Cardiff, UK (Sept. 2006); Krakow, Poland (Sept. 2008); Vienna, Austria (Sept. 2010) and the increasing desire to exchange ideas and experience is reflected in the number of delegates: 99, 108, 150, 100, 121, and 119. AE Testing in Europe is now well established in several important areas of proof testing, in-service monitoring, corrosion and leak detection. Development of the technology continues to be driven by the needs of the industry to reduce inspection and maintenance costs while preserving its assets and personnel safety. The advancement of PC technology has positively influenced new developments in hardware and software for powerful and user-friendly testing equipment. Sources: T.F. Drouillard, Adv. in AE-2007, pp. 1-10; H. Vallen, 26th EWGAE Proc. Vol. 1, DGZfP, pp. iii-v; Report for EWGAE 2006, Cardiff, UK

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The University of Granada The Universidad de Granada, founded in 1531, continues a long teaching tradition, the roots of which can be traced back to the madrasahs of the last Nasrid Kingdom. The University is a vibrant presence in the city of Granada, benefiting from the distinctive beauty of its environ-ment and a privileged geographical location. In Granada, there are four University Campuses, as well as the Campus Centro, which comprises all the faculties that had previously been spread throughout the historic part of the city. The UGR's policy of using buildings of historical and cultural value has enriched its heritage; it also has promoted the restoration and maintenance of these buildings. In addition to this emphasis on more traditional elements, the Health Science Technological Park, which is still being developed, demonstrates our strong commitment with innovation by promoting interaction with technological bio-health companies and favouring high-quality healthcare and biomedical knowledge. Over 60,000 undergraduate and postgraduate students study at the UGR, together with 20,000 students taking additional courses, language courses, summer courses etc. The University employs 3650 lecturers and over 2000 administration, technical and maintenance staff.

EWGAE Committee

Hartmut Vallen (Germany) - Chair Catherine Herve (France) - Deputy Chair Peter Tscheliesnig (Austria) - Secretary

Ireneusz Baran (Poland) - Treasurer Martine Wevers (Belgium) - Exec. Committee

Vaclav Svoboda (Czech Republic) - Exec. Committee

Conference Organizing Committee

Dr. Antolino Gallego (Chairman) Department of Applied Physics E.T.S. Ingeniera de Edificacin

antolino@ugr.es

Dr. Amadeo Benavent-Climent (co-Chairman) Department of Mechanics of Structures

E.T.S. Arquitectura benavent@ugr.es

Leandro Morillas (Secretariat)

Department of Mechanics of Structures E.T.S. Arquitectura lmorillas@ugr.es

Pedro A. Ortega (Webmaster) Consejera de Educacin

Junta de Andaluca informata@informata.es

Local Organizing Committee

Rafael Bravo, Enrique Castro, Cristobal Infantes, lvaro Ladrn de Guevara, Roberto Palma,

Joaqun Passolas, Liliana Romo, Francisco Sagasta, Antonio Snchez-Parejo, Elisabet Surez, Juan M. Vico

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Scientific Committee Anastasopoulos, Athanasios Envirocoustics SA Greece Baran, Ireneusz Cracow University of Technology Poland Brunner, Andreas EMPA Switzerland Carpinteri, Alberto Politecnico di Torino Italy Cole, Phil Physical Acoustics Ltd United Kingdom Damasceno, Sergio PETROBRASCENPES Brazil Enoki, Manabu The University of Tokio Japan Gallego, Antolino University of Granada Spain Grosse, Christian Technische Universitt Mnchen Germany Grum, Janez University of Ljubljana Slovenia Hamstad, Marvin University of Denver USA Herv, Catherine CETIM France Kuzmin, Alexey Interunis Russia Lenain, Jean-Claude MISTRAS Group, SA France Lopez-Pumarega, Isabel CNEA Argentina Mazal, Pavel Brno University of Technology Czech Republic Ohtsu, Masayasu Kumamoto University Japan Ono, Kanji University of California, Los Angeles USA Piotrkowski, Rosa University of General San Martin Argentina Prvorovsk, Zdenk Institute of Thermomechanics ASCR Czech Republic Proverbio, Edoardo University of Messina Italy Pullin, Rhys University of Cardiff United Kingdom Qi, Gary The University of Memphis USA Shiotani, Tomoki Kyoto University Japan Svoboda, Vclav Preditest Czech Republic Theobald, Pete National Physical Laboratory United Kingdom Tscheliesnig, Peter TV AUSTRIA SERVICES GMBH Austria Vallen, Hartmut Vallen Systeme GMBH Germany Wakayama, Shuichi Tokyo Metropolitan University Japan Yoshida, Kenichi The University of Tokushima Japan

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ing scheme, AE analysis and interpretation methods and evaluation guideline, enabling wide-spread AE applications of FRP vessel examination, in chemical industry in particular. Fowler [8e] documented success achieved in reducing major accidents via this AE method. In a re-vised document published in 1995 [10b], the intensity analysis method is used, relying on severity index and historic index, along with emission during load hold, total counts, high amplitude emission, Felicity effect as key elements of evaluation. Dr. Fowler states (e-mail, 2012) In the absence of CARP, the revised procedure became an open document. A number of organizations modified it for their own in-house needs. Some equipment vendors programmed it for use with their proprietary equipment. Even though not a public document, the technique has widespread use. His students extended its uses in FRP design and analysis [10c,d]. With improved FRP technology combined with the advances in AE instrumentation, numer-ous studies on research and applications followed and have been documented in AE Testing volumes of ASNT Nondestructive Testing Handbooks [12], where one can find many AE uses, such as for testing bucket-truck booms. Also available are six-volume conference pro-ceedings of AECM [13a-f], ASTM and ASME codes and standards, and many jour-nal/conference articles that reach into thousands. With his extensive composite research since early 1970s, Hamstad contributed important reviews [14a,b] along with his many papers. See also other notable reviews [8e,14c-e]. Gorman and Wevers [14f,g] reviewed AE methods for structural health monitoring (SHM) field. See [14h] for a review of newer AE analysis tech-niques. Early AE papers have been indexed in [15]. We must realize first that AE has not provided an effective NDT tool for composite industry. Such a tool is sensitive to existing defects, and is globally applicable to a large component in real time monitoring condition. While many success stories exist in FRP industry, AE has only played limited roles in many aerospace sites. We know AE can do better. In this paper, selected aspects of research and applications will be reviewed to highlight the progress achieved and to point to areas in need of more work. The main focus will be on development from the last 10-15 years since references cited above [12-14] have provided adequate cover-age. Hopefully, we can go to the next level of understanding to give better solutions. Applica-tions of advanced composites are expanding beyond aerospace and chemical industries into transportation industry and utilities. Such expansion presents renewed challenges for AE to be a worthy NDT method. We do have to overcome problems due to high signal attenuation in composites and need to explore the causes of Felicity effect and stress rupture, just listing a few. Collectively, we should resolve these obstacles, and make AE into a truly indispensa-ble NDT method. Research Fiber fracture From the early days of AE, glass fiber (GF) fracture has been recognized as energetic AE source detected close to the fracture load. AE from single fibers, tows (or strand) and unidi-rectional (UD) composites was initially characterized with amplitude distribution in order to compare with other failure mechanisms. By now, the difficulty of using AE amplitude by it-self in signal discrimination is well recognized. However, a misconception that fiber fracture always produces high-energy event still persists to this day and this section is intended to clarify the situation. For typical 10-m diameter UD GFRP, peak amplitude ranges from 60-80 dB (ref. 1 V=0 dB; literature in the 80s often used 1 V=20 dB and one needs to reduce amplitude values by

2

20 dB) [16,17]. This range depends on the fiber diameters and sensors used, but GF results are fairly consistent over the years when sensor difference is accounted for. Wolters [16a] used sensitive resonant sensors with less than 10-mm propagation distance and found 40-60 dB for 6 m, 65-85 dB for 13 m and 80-100 dB for 24 m fibers [16a], though another re-ported 50-70 dB for 13.5 m GF [16b]. In our test of woven GFRP [18a], high amplitude AE signals of 60 dB mean level were found at above 90% of the failure load, verified as fiber fracture microscopically. Woven GFRP failure mechanisms are the subject of a detailed study [18b], and complementary AE study should allow better understanding of woven com-posites, which are increasingly used in aircraft fuselage with carbon fibers, such as Boeing 787 (A.K. Mal, July 2012, private communication). Recent studies [19a-c] reexamined single fiber composite (or SFC, with epoxy matrix) tests. Nordstrom [19a] used 18-m glass fibers and found peak amplitude of fiber fracture at 80 dB (150 kHz sensor). He also obtained Weibull moduli of 7-9 and correlated the shape of stress-strain curves to progressing fiber fracture [see also 22d]. Bohse [19b] used 20-m glass fibers with peak amplitude of fiber fracture to be 58-81 dB (PAC-WD sensor used). More recently, de Oliveira [19c] conducted careful SFC tests determining the amplitude of fiber breaks along with source location data. Such amplitude correction with source location was introduced earlier [19d], but has seldom been used since. Carey [19f] also introduced distance correction in his AE signature analysis work on CFRP. Using 21-m GF, SFC showed 20-dB attenuation over 45 mm and fiber-break AE mean amplitude of 73 (53-79) dB (58 dB for 11-m GF); the lower values are due to the use of a wideband sensor (DWC B1025). In single tow 9-m GF composite experiment, Nanjyo et al. [21a] obtained lower amplitude values ranging from 46-56 dB (Pico sensor). Diameter and sensor differences are responsible for the observed variation in amplitude rang-es since GF strength and modulus are essentially unchanged. Berthelot obtained comparable amplitude range for 13-15 m GF and for 7-m T300 CF [21b,c].

Figure 1 Peak amplitude distribution of fiber fracture at 40-60% of the fracture load from 9-ply UD sample with pre-cut mid-plies [22a]. Celion G50 fiber. When carbon fibers (CF) were tested initially, peak amplitude of AE signals for T300 CF was 30 dB less than glass fiber [16b,c]. In part, this is because most carbon fibers have 7-m di-ameter, and earlier CF strength was lower (T300 tensile strength = 2.65 GPa circa 1982). In our study [22a,b], the peak amplitude range was 30-50 dB for Celion G50 UD composites of various cut-ply arrangements (MAC175 sensor). This is 10 dB lower than GF results [16a]. These AE signals from CF started to occur at 25-33% of the composite strength and also had short duration (mostly

ter) CF as well, giving comparable amplitude as GF data cited above; 44-56 dB for M50 and 38-58 dB for XN50. These CF were stronger than T300 or G50; 4.2 GPa for M50 and 3.7 GPa for XN50, accounting for the higher amplitude by nearly 20 dB (Fig. 2). In this study, the Weibull analysis of CF fracture strength and AE hits-strength distributions was reported. For three lots of CF, Weibull moduli of 5.7 to 13 were found, while AE hits had higher Weibull moduli, m = 7 to 35, indicating multiple CF fracture in the tow tests [21a]. A recent statistical analysis of AE signals from non-bonded GF bundles showed m to be 2.8-4.0 for 15-m diameter fibers [22d], which are about a half of m = 5.6 to 9 in tow tests [19a,c]. Recently, a new method for composite fracture study appeared [20a,b]; this is High resolu-tion Synchrotron Radiation Computed Tomography (SRCT) and was used to capture fiber damage progression in a cross-ply CFRP laminate. [20a] showed that transverse ply cracks initiate at 20% of the failure load, whilst 0-splitting initiates at ~40% and large delamina-tions at ~60%. It is important to note that these cracking events occur in advance of signifi-cant fiber breaks that start after 60%. In [20b], fiber fracture was analyzed and compared with other studies, including AE. AE results for fiber fracture accumulation showed the same trend as SRCT and Weibull moduli are similar (also similar to [19a,c, 21a]). The number of fiber breaks (Nf) was found to follow

Nf = C (/)m, (1)

where is stress and is characteristic stress and m is Weibull modulus. See also [19a]. It is hoped this method can be used widely leading to more definitive AE mechanism studies.

Figure 2 Peak amplitude distribution of fiber fracture of a) XN50 pitch CF, b) M50 PAN CF, c) E-glass fiber tow-composite samples. Pico sensor. [21] SFC tests with 7-m carbon fibers [19b] showed amplitude of fiber fracture at 45-75 dB with the median value of 65 dB (WD sensor used; fiber unidentified, but ~late-90s vintage), com-parable to usual GF fracture amplitude. In this work, CF diameter is ~1/3 of GF (18 dB in area) and less than 10 dB amplitude difference implies that higher CF modulus (by a factor of 3.5) accounted for the comparatively stronger AE in CF fracture (assuming comparable frac-ture strains). This CF SFC fracture result is 20-30 dB higher than previous studies from the 80s [16b]. This higher range of AE amplitude for CF fracture seems to reflect higher CF strength (2.5 GPa for G50 to 6.3 GPa for Toray T1000) and higher fracture strain values (0.7% for G50 to 2.2% for T1000). This is obviously due to dramatic improvements of CF properties in the last three decades. Thus, if one uses newer CF fracturing at 3-5 GPa (50-80% of T1000 fiber strength) in lieu of the G50 CF fractured at 350-530 MPa in 1988, 40 dB stronger AE signals are expected. Thus, we should observe CF fracture amplitude of similar or higher magnitude as 10-m GF. In fact, Sause et al. [33b] showed this recently. Compar-ing E-GF (6.5 m) and T800-CF (5.2 m), he obtained fiber fracture energy of 12.6 and 13.1 mNm3/s2. Assuming similar signal durations, the amplitude is expected to be comparable for comparable fiber diameters. It should be clear that AE amplitude from fiber fracture can vary

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widely. Small diameter, low strength CF produces AE amplitude comparable to the matrix sources (in 30-40 dB), while larger diameter fibers lead to higher amplitude to 80 dB. For thicker filaments, it can be even higher; AE signals from 150-m boron filaments produced 95+ dB signals [22e]. Fiber optic samples also give similar strong AE. Recent tow composite tests using T1000 CF indicates the number of CF fracture at stresses below 60% of the fracture strength is grossly reduced with improved CF quality (no ampli-tude data was reported unfortunately) [23]. Figure 3 shows this behavior in tensile testing with intermittent load hold (ILH) for characterizing Felicity ratio, but cumulative hits start to rise rapidly after the fifth load hold or at 213 lbs (failed at 352 lbs) or 60.5% of 6.77 GPa fracture strength. The same NASA group examined Kevlar tow composites as well. In this case, fibers start to fail at ~90% of the fracture strength [24]. This behavior is in common with GF composites. Detailed analysis of these experiments of UD composites should clarify this situation on amplitude levels.

Figure 3 AE of T1000 CF tow-composite test using ILH schedule at NASA. Red: Load (lbs), Blue: AE. [23]

When a carbon or glass fiber fractures, it does so in brittle manner. Typical crack velocity in brittle fracture is several tenths of mm/s or higher (e.g., 0.52 mm/s for PMMA [25] and 1.6 mm/s for glass [26]). Actual cracking velocity is expected to be much smaller as a static crack needs to accelerate under stress. For GF and CF, fracture time is expected to be 0.7~2 s assuming crack velocity of 5~10 m/s (or m/s). The lower velocity limit is for GF [26], while CF crack velocity is twice that since CF moduli are ~4 times that of GF (crack velocity scaling with the Rayleigh velocity [25]). This translates to the center frequency of 0.5~1.4 MHz for AE signals from fiber fracture, which is partially within the high frequency range of common AE sensors, such as PAC WD, Pico and DWC B1025/B1080. Indeed, Gorman [27] reported such fiber-break signals from a COPV cover the entire range of flat sensor response (20-400 kHz), in contrast to matrix cracks (splitting), for which detected signals had frequen-cy contents confined below 50 kHz. He also observed that, in hundreds of pressure vessel tests, including both CFRP and FRP vessels, waves with the highest frequencies appeared as the load or pressure level exceeds above 80% of ultimate stress. Thus, tow composite behav-ior extends to the windings of COPV. SFC samples in de Oliveiras study also showed high-

5

frequency components [19c]. His signals contained strong 300-1050 kHz components (using DWC B1025), especially when debonding accompanies a fiber break or when 7-m CF SFC has a fiber break. He attributes the effect of debonding to the higher freedom of the fiber ends at fracture. In contrast, GF fiber break signals without simultaneous debonding (11 and 21 m GF) have the highest peak at ~170 kHz with gradually decreasing high-frequency com-ponents. Gorman [27] observed similar vibration-induced AE from broken ends of tows and further clarification of debonding may be worthwhile in providing new inspection scheme. It should be noted here that Kim and Nairn [19e] showed that fiber-matrix debonding always accompanies a fiber fracture in GF and CF SFC. Their epoxy resin (Epon 828) did not exhibit flat or conical fracture envisioned by earlier work [16b]. We must have independent proof before assigning to an AE cluster a mechanism of fiber fracture without debonding. When using broadband sensors, the observed peaks in the frequency spectra have to be treat-ed with caution. Unless one has broadband flat-in-frequency sensor and propagation path, the peak frequency differs from the source-defined frequency. Most available sensors today have limited frequency range and have resonance peaks. Sheet or plate geometry has preferential frequency ranges of wave propagation. For example, Bohse [19b] correlated fiber fracture to 500-kHz peak using WD sensor while matrix fracture contains lower frequency peaks only. This work and many others clearly demonstrated that one can often identify AE mechanisms via the frequency domain information. In fact, it is an important part of pattern recognition analysis on composite AE [28]. On the other hand, one cannot conclude fiber fracture has such a frequency peak because the WD sensor used is the origin of this peak (WD has three main resonances including one at 500 kHz [29]). Such arguments assigning frequency peaks to some specific AE sources have shown up too often in the past [30a-c], yet most such cases revert to specific sensor or sample resonances and sensor-source path has significant effects. For example, de Groot et al. [30b] examined a variety of CFRP and assigned frequency rang-es to specific mechanisms. Such correlation is valid for the test condition they utilized, but the particular peaks originated from the use of WD sensor again, which was also the sensor used in Giordano et al. [30c]. Eaton et al. [30d] warned that the overriding effects of speci-men geometry and sensor response can produce misleading results in their study to use fre-quency for separating source mechanisms having the differing relaxation time. One should not expect specific frequency peaks in other test arrangements, except for the general concept of faster source events to have higher frequency contents. It is instructive to refer to the most broadband AE study reported to date by Scruby and Battle, extending the limit to 3 MHz (6 dB point) [31]. They did find two peaks for the signal from fatigue crack propagation, the main one at 600 kHz. However, these peaks are due to the normal modes of the sample. Even more broadband system is needed to identify the key frequency of fiber fracture via the fre-quency spectrum analysis since fracture time is quite short, on the order of 0.1 s.

Figure 4 Waveforms of fiber fracture (theory-broken line; experiment-solid line). [32]

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This situation regarding the frequency peaks also arises in theoretical studies. For example, Guo et al. [32] obtained the solution of the wave motion produced by a general source in a composite plate using laminate theory with transverse shear correction. He identified the dis-tinguishing features of the wave motion produced by various microfracture events (e.g., ma-trix cracking, delamination, fiber break, etc.) in thin composite laminates. Guo represented fiber fracture with 0.125-s cracking time in a model CFRP plate among others. Using the experimental transfer function of a broadband sensor (B1025) showing 200-2000 kHz band-width (within 3 dB of the peak at 1 MHz,), the calculated waveform for CF fracture shows dominant ~1 MHz oscillations. See Fig. 4. The apparent frequency in this waveform is much lower than the expected center frequency of the original crack model, or 8 MHz. Note that Guo did not claim fiber fracture AE to have 1 MHz peak frequency. Recently, Sause [33] used finite-element modeling of laminated composites and crack models for CF fracture (in-plane) and resin fracture (out-of-plane) with 0.1-s cracking time. The CF crack produced displacement signals dominated by ~1-MHz So waves (where So velocity starts to decrease sharply), whereas the resin crack produced 100-300 kHz Ao waves. His model included Lamb wave propagation of 50 mm from the source. Here, the Lamb wave dispersion effects play the central role in limiting the upper frequency limit of the simulated AE signal and de-ciding the centroid frequency. Sause has presented more extensive analysis of related model-ing in his thesis work [34]. His results point to the additional factor we must consider in try-ing to assess the source function when AE moves through thin sheets/plates. [It is noted that Ichikawa et al. [A1] reported theoretical calculations for the intensity of Lamb wave modes and dispersion effects as a function of frequency-thickness product. They used steel as the medium, but confirmed the Lamb wave behavior observed by others. See Appendix 1.] In considering the frequency contents of AE signals, one must not forget that high frequency components are attenuated rapidly as the signals travel from the source to sensors. Unless the sensors are almost over the source, the high frequency components above 200-300 kHz are reduced to background beyond 0.5~1 m distance. This has been known for 30 years, but we have no effective remedy applicable to typical applications. In laboratory, sensors are close to sources and we can try to understand the basic behavior.

Figure 5 Observed and matched calculated waveforms with source parameters. UD GFRP test. [35]

Another approach to the study of fiber-AE originated from Takemoto group. Suzuki et al. [35] conducted elaborate experimental analysis of GF fracture, deducing the source function

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for detected AE signals during tensile testing of UD FRP. The goal of getting the force-time function of an AE source is similar to the deconvolution approach from the 80s, but wave-form simulation and matching of source parameters distinguish this method from the past at-tempts [36]. Suzuki [35] incorporated the viscoelastic nature of FRP by introducing a fre-quency independent relaxation function, R(t), after Weaver [37] and extended the Greens function approach of Ohtsu and Ono [38]. Suzuki [35] used a NIST-type displacement sensor, as well as an 8-channel source location system that identified the source position. Both dis-placement waveform and location data were fed as input to waveform simulation algorithm. Adjusting the source rise time and crack volume, theoretical and experimental waveforms were matched, where the latter was obtained by a conical displacement sensor of NIST de-sign derivative. Examples of such pair are shown in Fig. 5 for fiber-break cases. A cross plot of source parameters, crack volume vs. rise time, in Fig. 6 indicates the crack velocity limits of 2-22 m/s. The median crack velocity is 6.5 m/s in a good agreement with the measured crack velocity in glass of 5 m/s [26]. Note that, in elastodynamic theory [37,38], the source parameter representing its magnitude is given by the product of the source area (A) and displacement (bi), or crack volume. In fiber fracture, fiber opening equals bi, not the released elastic energy. Thus, the energy analysis given in the past, such as by Lorenzo [16b] and Berthelot [21d] needs to be modified. Anoth-er overlooked aspect of AE signal analysis is the modeling of sensor input. For example, Berthelot [21d] used a rectangular pulse to represent a crack signal, which is a sensible first-order approximation for surface displacement due to an opening crack detected at the epicen-ter. Note that the source displacement function is step-like, but with a slope (Fig. 4 in 21d). Because Greens function is convolved with the source [38], surface displacement becomes a Gaussian-type mono-pole pulse (left: Fig. 5, 21d). It was simplified to a rectangular one (right: Fig. 5, 21d). AE sensors, however, typically respond to the velocity component. Thus, the sensor responds to two sharp pulses of opposite polarity separated by the duration of the rectangular pulse (Tf). In this case, the FFT of the velocity response has the center frequency of 1/2Tf, instead of the displacement response that peaks at zero and vanishes at 1/Tf. When one uses a Gaussian shaped displacement pulse (a half-width of 0.22 s; Appendix 2, Fig. A2a), its FFT gradually vanishes without multiple zero points as can be seen in Fig. A2b. Note the half-width is 2 MHz, equaling the inverse of the base width of ~0.5 s in Fig. A2a. The derivative of the Gaussian pulse is known as a Gaussian monopulse and is shown in Fig. A2c. The FFT pairs of Gaussian monopulses of 0.2 to 2 MHz are shown in Fig. A3. It is evi-dent that these pulses are broad and one can hardly use the spectra for source discrimination. Even recent AE modeling efforts have not taken these points raised here and reevaluation is called for.

Figure 6 Crack volume vs. source rise time. Data points are within the zone between crack velocities of 2.3 and 22 m/s. [35]

Figure 7 Typical waveform from a transvers matrix crack in [0/90n/0] CFRP. [39]

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Matrix and interfaces AE signals originating from the matrix and interfaces are varied and range from weak to strong, making unambiguous characterization difficult in practical composite structures. Sev-eral different mechanisms are expected to overlap as well. In model composites, however, clear distinction is possible. Transverse matrix cracks: Prosser [39] reported one-to-one correlation between observed So Lamb wave signals and transverse matrix cracks in cross-ply composites (Fig. 7) when 90-ply thickness exceeds 0.4 mm (3 plies). Notice that this signal is of high-frequency type (~700 kHz) and in thick cases (>6 plies) of very high amplitude (>80 dB). Gorman [40] showed even stronger events in (0/90)s CFRP, ranging up to 98 dB, but with >100 s dura-tion. Some of these signals with short duration are hard to differentiate from fiber-break sig-nals. When transverse matrix cracks occur, three possible micromechanisms exist; i) matrix (resin) cracking, ii) debonding of fiber-resin boundaries and iii) fracture of bridging fibers. Matrix is low strength and slower fracture produces only low energy (amplitude) AE. See [19b] for epoxy cracks having ~55 dB AE even for large area cracks. Matrix cracks are pre-sent in notched woven-roving fracture surfaces in [18a,b] as resin-rich areas of the size of roving diameter exist throughout the composite. In laminates made from UD prepregs, only the interlaminar layers have contiguous resin constituent and the contribution of mechanism (i) is limited. Debonding is expected to be the main mechanism for the transverse matrix cracking in laminates with straight fibers and as the source of first ply failure. Mechanisms i) and ii) typically coexist in woven composites [18a,b]. Woo and Choi [41a] characterized debonding AE to have low amplitude (under 70 dB) and low frequency (lower frequency peaks than the sensor resonance of 265 kHz for PAC 30) using in situ microscopical obser-vation of the notch tip zone. They showed, however, that once macrocracking begins, higher amplitude/frequency (>70 dB, 430-450 kHz) peaks are dominant, implying bridging fiber failures contribute substantially. Fracture surface showed both of these (ii and iii) mecha-nisms. Elevated temperature exposure (wet or dry) affects the matrix-bonding behavior, which was detected with AE. Schoig et al. [41b] used environmental SEM to assess damage mechanisms of short-fiber GFRP and identified 14 different mechanisms and recorded AE simultaneously. However, too many processes occur together. No clear correlation to AE signal characteristics is established so far. Bussiba et al. [41c] exposed CFRP (0/10 layup) to 100C for 2 weeks and saw accelerated debonding with the knee strain down from 1.5% to 0.6%. Wet exposure of GFRP was reported in [28, see also 84].

In de Oliveiras thesis [19c], he summarized nearly 20 other AE works on transverse matrix cracking and also reported own cross-ply GFRP testing [see also 19e]. He developed artificial neural networks based on self-organizing maps with waveform and wavelet-transform data as input. Among six clusters identified, transverse matrix cracking produced two types of AE signals having characteristic So waves centering at 350-400 kHz lasting 30~60 s. The low frequency part arrived first as expected for So waves. Shorter duration signals (type II) are attributed to the initiation, while longer ones (type I) to the propagation of transverse matrix cracks [19e]. Godin et al. [42a-d] made a series of well-designed studies using UD, angle-ply and cross-ply composites with advanced pattern recognition analysis. Using Kohonen map and k-means methods, they identified four distinct signal types and attributed them to matrix cracking, interfacial debonding, fiber fracture and delamination. Waveforms of the 2nd and 3rd types are visually indistinguishable, however. The use of Kohonen maps appears effective in identifying clusters of AE signals and will be examined in a later section.

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Delamination-Shear: Ono [22] reported AE from the delamination of UD CFRP with pre-cut plies with several different shearing zones. Shear strength level was 5 24 MPa. Overlap length ranged from 25 to 75 mm (13 mm width). Tensile loading produced initially medium amplitude signals (50 - 70 dB) with an average event duration of ~120 s from the initiation of Mode II delamination. At higher loads, high amplitude events (70-130 dB) of long (> 200 s) event duration were found. These are caused by rapid advances of delamination under mainly Mode II loading with out-of-plane stress near cut edges. Lagunegrand et al. [43] ex-amined free-edge delamination on 30 angle ply CFRP and identified 90-100 dB signals as Mode-III delamination induced AE. This delamination occurred between +30 and 30 plies in shear, while mixed mode shear was reported in [15n/90m/-15n]s laminates. Scholey [44] examined delamination in large-sized QI CFRP samples with cut plies, getting 10-50 pm Ao-wave amplitude as the area of delamination reaches ~30 x 50 mm after loading. Surprisingly, duration is only ~100 s, implying delamination expands in small steps. Ono and Huang [28] examined waveforms of various AE origins of CFRP tensile samples (UD, cross-ply, quasi-isotropic) in conjunction with pattern recognition analysis. One of six types identified is due to delamination, which becomes active just prior to sample failures. As shown in Fig. 8, type-D delamination signal has the lowest frequency content (~100 kHz) and long duration, while fiber-break AE (types A and B, latter with larger matrix debonding) has highest frequency content and short duration. (Type C is from transverse matrix crack and type E from split-ting.)

Figure 8 Waveforms of 6 different types, classified by pattern recognition analysis of AE signals from QI CFRP laminate. Time scale: 41 s/div, Amplitude scale varies. WD sensor at the sample center. [28] De Olveira [19c,d] identified two types (IV and V) of delamination-induced AE signals in cross-ply GFRP tests. Both are Ao-mode waves with low frequency contents under 400 kHz (down to 20 kHz in type V) and strongest contents at 150-230 kHz. Type IV was observed at lower loads, attributed to matrix-fiber decohesion as well as delamination. Type V appeared near final fracture when delamination activity was high.

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Delamination-Opening: Bohse and coworkers [45a,b] studied Mode I (crack-opening type) delamination using DCB specimens and correlated AE energy with strain energy release rate (GIc), supporting the concept of damage zone concept ahead of the crack tip. They consider the presence of matrix cracking and matrix-fiber interfacial debonding, the latter in stronger interface condition. Some bridging fiber fracture is possible as well as in [41]. This also has more high frequency component, allowing the discrimination of the two mechanisms. In both, however, the main frequency feature is the dominant low frequency components below 200 kHz. Amplitude of these AE is mostly (~90%) in 60-80 dB range. Another CFRP DCB tests [30b] reported essentially same results with a comparable low frequency peak (120 kHz with minor one ~300 kHz, both characteristic of WD sensor). This low frequency peak was also seen in epoxy resin crack and in lap-shear AE. Recently, Sause et al. [33b] published a comprehensive study of DCB tests with AE analysis examining interlaminar crack propagation in GFRP and CFRP. They conducted pattern recognition analysis to identify three clusters, corresponding to matrix cracking, interfacial failure and fiber breakage, using mostly frequency-based features, such as average, centroid, initiation, peak, weighted peak frequency, six partial powers, etc. Cross plot of partial power at 450-600 kHz vs. weighted peak frequency easily separated the 3 clusters. Figure 4 in [33b] compares observed and FEM-simulated signals and wavelet spectrograms, showing good agreement. Interfacial failure dominates cumulative signal amplitude, followed by matrix cracks. Fiber breaks contribution is low as this comes from bridging failure. This theory-test-model hybrid approach holds great promise in assessing complex structural composite design under various loading conditions, integrating AE as a part of health-monitoring scheme.

Figure 9 Amplitude vs. duration of tow-composite samples with (a) no and (b) maximum surface treatment. [21a] Fiber-pullout: In tow composite tests, Nanjyo [21a] used 4 different levels of (pitch-)CF sur-face treatment. Without treatment (level 0), many fiber pull-outs of up to 100 m were ob-served after failure. With maximum treatment (level 5), fiber-matrix bonding improved and few fiber pull-outs were found producing flat fracture surface. Comparison of these two con-ditions on amplitude-duration plots indicates events with 60-80 dB amplitude, longer than 80 s duration increased more than ten-folds in the level-0 sample. These long duration signals are due to fiber pull-outs. This means pull-out AE amplitude is 9 dB higher to fiber fracture (Fig. 9) on average. However, differentiation must rely on duration. Single glass-fiber pull-out experiment by Mielke [46a] produced similar results regarding amplitude, though fiber diameters of 20-25 m were used and fiber-break amplitude was ~10 dB higher than pull-out

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signals while another study examined pull-out of SiC fiber from glass matrix [46b]. A more recent work used AE in single Kevlar-fiber pull-out tests, and showed energetic signals of 50-90 dB (Pico sensor). However, AE part is difficult to evaluate lacking key details [46c]. Mixed mode cases: Mizutani et al. [47] examined fracture of cross-ply CFRP under central point loading and observed four separate types of AE signals. Signal simulation using laser sources allowed him to establish source mechanisms of fiber fracture, transverse crack, de-lamination (shear) and matrix crack, designated as type-A, -B, -C and -D. These 4 signal types are shown in Fig. 10 together with wavelet spectrograms. Two sensors placed at 90 angle from the loading point (along 0 and 90 relative to the surface fiber axis) showed vast-ly different waveforms, helping the identification process. Of 327 total, initial 70 events showed the following: Mode-I type fiber fracture in the front layer (the first event + 3), fol-lowed by transverse matrix cracks in the mid-lamina (27), from the middle part on, delamina- tion (15) and splitting-type matrix cracks (11), (13 unidentified). They also studied the same experiment, but under dynamic loading condition. Details can be seen in JAE [48]. Four types of cross-ply CFRP plates were impacted. Lamb-wave AE signals were detected by small AE sensors on both surfaces to separate So and Ao modes. Only Impact-AE is obtained when the impact energy is below a fracture threshold. When internal fracture occurs, both Impact-AE and fracture-induced AE (or Fracture-AE) were detected. Most of strong AE sig-nals were correlated with splitting matrix fracture source (type D in Fig. 10).

Figure 10 Waveforms and wavelet spectrograms of 4 types of signals from point-loaded cross-ply CFRP. [44]

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There are other types of matrix fracture. For example, matrix fracture similar to the trans-verse matrix crack, but fibers oriented along the loading direction, also occurs. This is usually called splitting and corresponding waveform is that of type E in Fig. 8 [28] (also type III in [19c,d]). Another type of matrix fracture occurs under shear stress along the fiber-matrix in-terfaces in angle plies, such as those 45 plies in quasi-isotropic laminates. This has a char-acteristic waveform of type F in Fig. 8. Both E and F types have slowly rise time, reflecting slower growth of such cracking (WD sensor was on top of these sources 0.5 mm away, so no propagation induced slow rise time). The characterization of signal types relied on the lami-nate types and when these signal types were emitted during loading. (Note these features are dependent of test conditions: these tests used thin (~1 mm thick), narrow (13 mm wide) CFRP samples with WD sensor attached at mid-section. See also [49].) Felicity effects When damage accumulates in materials, Kaiser effect breaks down. This is usually character-ized by Felicity ratio; the stress of AE restart/prior maximum stress. Felicity ratio has been linked to the residual strength of composites, and has been studied over many years: see Hamstads thorough discussion in 1986 [50]. Changes in Felicity ratio on laminates are shown in Fig. 11a and b. Hamstad [50] shows in Fig. 11a a schematic plot of GFRP data by Fowler and Gray [51], where it drops with applied load. Awerbuchs data on a multi-ply CFRP decreased with increasing prior stress above 1/3 of the tensile strength of 272 MPa [52a]. Felicity ratio was down to 0.7 with prior stress of 220 MPa or 80% of the maximum. These are data from tensile coupons. On bend testing, GFRP also exhibited Felicity effect, but stress level at the start of decreasing Felicity ratio is higher (75-80%) instead of 45-60% in Fig. 11a, as shown in Fig. 12 [52b]. Obviously, the difference arises from different failure mechanisms between tension and bend tests, but exact cause is unknown. AE monitoring in compression-after-impact or bending tests with damages showed the reduction of FR values [e.g., 53a,b]. Various AE parameters including FR were combined with neural networks for burst pressure prediction [e.g., Hill, ref. 12, pp. 382-387]. The origins of Felicity effects need to be explored further as we find few articles beyond Hamstad [50]. It is obvious that the stiffness of fiber-damaged zone is reduced and stress/strain differentials develop against the surrounding sound zones upon reloading. This is realized during any unload-reload step [cf. 22d], since the stiffness is proportional to the frac-tion of intact fibers. A damaged zone has a lower stiffness and deforms more. The stress/strain differentials, in turn, produce additional interlaminar stresses, leading to prema-ture delamination. This can be the source of Felicity effect in impact-damaged vessels. In the absence of fiber damage, delamination itself should give rise to frictional AE during an un-load-reload step. At any interlaminar, resin-rich layer between plies of different fiber orienta-tions, interlaminar shear stresses are always present. While the shear fracture strain is of the order of 1% in most resins used today, it is common to use longitudinal-hoop fiber angles of 60-70 and substantial interlaminar shear stresses result upon loading. With our current AE methods, separating delamination and frictional AE signals is feasible. With the above sce-narios for Felicity effect, we should expect strong FR dependence on lay-up sequence. Felicity ratio (FR) provided sensitive measure of developing damage under compressive tests [54]. CFRP laminates with 914 or PEEK resin matrix having a seeded defect were cyclically loaded and f = 1 FR was used as damage indicator (of internal delamination). Applied load P and f are related with laminate specific manner with high R2-values >0.98. See Fig. 13. As the marks for visible crack indicates, f-value gives the warning of internal damage.

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Figure 11 a) FR vs. applied load of GFRP [50]. b) FR vs. applied stress of multiply CFRP [52a]

Figure 12 FR vs. stress ratio of a GFRP under bending load [52b]

Figure 13 Cyclically applied peak load vs. f = 1FR for CFRP. a) with different resin, b) different orientation [54] Loading is compressive and CFRP has an internal defect to initiate delamination. Appearance of macro-cracks is marked by squares. Several stressing sequences are used in obtaining FR. The usual one is to unload from the previous maximum as in [54], whereas incomplete unload sequences are more convenient to perform. One such example is shown in Fig. 3, called intermittent load hold (ILH), while CARP procedure calls for 10% unload with stepped loading [Fig. 4, 10a]. The origin of Felic-ity effects is most likely from frictional (secondary) AE [52a], but the link between Felicity effects and residual strength is empirical. Toward clarifying this link, NASA has embarked on extensive basic study of FR as they view this as one key element in the prediction of com-

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posite over-wrapped pressure vessel (COPV) performance. Specifically, it was found that some COPV failed in accelerated stress rupture testing; that is, COPV is pressurized and held for extended periods. They cannot afford such a failure in space [55]. Tow composite tests using Kevlar 49 and IM7 and T1000 carbon fibers were initiated with 4 sensors mounted on 250 mm long samples [23,56-58]. AE results for a T1000 tow test was seen in Fig. 3. FR from this and similar tests showed quite different behavior compared what we have observed previously. Figure 14 shows the new findings. Data for an IM7 COPV is also included for comparison. Felicity ratios given here were determined using the first AE event for Kevlar-epoxy tow, and the mean of the first 15 events for T1000 tow, IM7 tow, and the single IM7 COPV. FR values are in all 4 cases higher than 0.9 even very close to the failure load. FR is above 1.0 up to load ratio of 0.6-0.7 for CF and 0.85 for Kevlar fiber. For COPV, FR stays above 1.05 to load ratio of 0.78. First, these finding again imply that fibers remain intact to much higher stress levels than older CFs like G50 used in [22a] (IM7 is relatively old, how-ever) as discussed earlier in conjunction with Fig. 3. A recent study [59] of T1000 Weibull modulus indicates that m-value at lower strength range exceeds 30 (overall m = 5.9), thus virtually eliminating fiber fracture below 80% of the average strength. Second, frictional mechanisms are not as active as in old CFRP, reflective of better interfacial bonding. Inter-laminar shear strength has doubled from previously common AS4/3501 (76 MPa) to newer T1000 CFRP (150 MPa). This condition is true even in COPV, in which windings of various orientations are expected to lead to interlaminar shearing and friction (it is unclear if this par-ticular COPV was subjected to autofrettage* (*an over-pressurization step to put the metal liner in compression), which can suppress AE at lower loads.) Third, flaws like splitting or delamina-tion are not expected in tow composites, although this is not the case for COPV. As higher FR values are linked to low damage state and higher failure strength, more extensive studies on composite Felicity effects seem to be highly advisable. These should point to improved materials and manufacturing processes. FR on pressure vessels will be discussed later.

Figure 14 FR vs. load ratio for T1000 carbon-epoxy tow (black-dotted), IM7 carbon-epoxy tow (grey-dashed), Kevlar 49 -epoxy tow (white-solid lines), and an IM7 composite overwrapped pressure vessel (large crossed hexagonal symbols). 95 % confidence intervals also shown. [58]

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Wave propagation Typical composite structures comprise thin members and AE signals are transmitted as Lamb or plate waves of extensional (or symmetric) and flexural (asymmetric) modes. Many com-posite members are anisotropic as well. Basic aspects on plate waves have been covered well in [12] and standard textbook [60]. In composite AE field, work by Gorman and coworkers [61a-c] brought the focus on the peculiar wave propagation behavior of AE signals in thin composites, especially on the aspect of source orientation producing different wave modes. Hamstad and his NIST colleagues enhanced our understanding with extensive finite element modeling studies to shed light on the source types and resultant plate waves [62a-d]. Large wave attenuation in composites has been accounted for in CARP methods [8-12]. Downs and Hamstad [63] showed the sharp amplitude drop (20 dB in 6 mm) due to geometrical spread-ing in CFRP vessels of inverse-square-root distance dependence. More recently, the aniso-tropic nature of composite plates has been included in analysis. Mal et al. [64] used a classi-cal damping factor approach to anisotropic wave analysis and found satisfactory results be-tween theory and experiment (though this work is difficult to transfer to AE study). Sause incorporated suitable source models and wave propagation analysis on such CFRP model sheets [33,34]. Using complex elastic modulus obtained by air-ultrasonic technique [65a,b], some wave propagation analysis included attenuation factors as well [66-69]. Anisotropic velocity calculations are also reported for common CFRP layups [70-72]. Actual measurements of attenuation are limited. Prosser [39] obtained apparent attenuation of So and Ao waves in IM7/977-2 CFRP of 1.2 and 3.7 mm thick quasi-isotropic laminates. For So mode in the thinner plate, attenuation was 42 dB/m and for Ao mode it was 83 dB/m. For the thick plate, corresponding values were 35 and 51 dB/m, indicating the effect of thickness. Attenuation observed on a CFRP pressure vessel [63] was higher. These values change when geometrical spread of inverse-square-root is accounted for. Gallego and Ono (see pp. 573-580) report attenuation measurements on CFRP plates of three types of layups. So and Ao wave modes are separately evaluated and frequency effects are examined. It is evident that any AE analysis must consider frequency-dependent signal loss through the transmission. Applications Pressure vessels and tanks - 1 When AE applications mature, we find few reports in publications especially after these are compiled in reference books, like NDT Handbook [12] and codes and standards established. Applications to chemical tanks and vessels fall into this category after the first three AERC/AECM conferences [13a-c]. Rocket motorcase applications are at a similar state [see ref. 12, pp. 377-381]; actually, few reports had appeared from the beginning due to their sen-sitive nature. In this connection, it is interesting to find a review on the burst pressure predic-tion; Joselin [73] covers the subject quite comprehensively, introducing each paper adequate-ly without inserting own interpretation. Thus, a reader must judge which approach is success-ful/unsuccessful on his/her own. Also many papers from AECM series conference proceed-ings are missing, as access to them may be difficult outside the US. Still this review is a rec-ommended reading on this topic. There are two important issues at this time. One is the evaluation of impact damage and the other is estimating the residual life of COPV. The first topic is not limited to pressure vessels, but it is one of the critical problems in aerospace circle. This has been worked with AE for more than 20 years [e.g., 74-80]. It is known that impact energy links to the burst pressure. Many AE parameters have been successful in predicting the burst pressure [12,73]. An

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example of burst pressure-Felicity ratio correlation is shown in Fig. 15 [75]. While the sam-ple size of this study is small, it is believed a larger database has been prepared to show the same trend within the aerospace circle. Yet industrial practice still uses AE as a supplemen-tary method. As a tool for space COPV inspection, Chang [81] in 2003 assessed AE to be better on inspection time and field use, as average on flaw characterization, simplici-ty and COPV preparation and as weak on whole field, data evaluation and sensitivi-ty. In the AE field, we think whole field, and sensitivity as AEs major strong points, but actual judgment by the practicing engineers is much more critical. As mentioned in Felicity effect section, NASA is attempting to use AE for COPV burst pres-sure prediction [55-58]. This is now based on the higher levels of newly measured FR values of near unity. Figure 16 shows Felicity ratio with increasing previous highest pressure for an IM7 COPV during two separate pressurizations to 6800 psi (circles) and to burst (crosses). A linear fit is made to both pressurization data. Once the FR value at burst is established with confidence, this will allow burst pressure prediction from lower pressure ILH loading sched- ule. As noted before, this FR-pressure relation is comparable to tow composite behavior. This surprising similarity is promising in a sense that newer COPV manufacturing is eliminating many sources of flaws that previously decreased FR values, and also reduced COPV strength. It is hoped to see even better correlation develops in the case of newer T1000 fibers.

Figure 15 FR values are almost linearly related to failure strength of spherical Kevlar COPV [75].

Figure 16 Felicity ratio vs. increasing previous highest pressure for an IM7 COPV. Data from [58].

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In connection to the use of FR values, it should be noted that another parameter is useful in some cases. Downs and Hamstad [78b] defined Shelby (countdown) ratio, which is (the load during unload when a predefined AE activity level is reached) divided by (the previous max-imum load). This ratio was used much like FR values in correlating to the burst strength of CFRP pressure vessels. This ratio is being used in the NASA study discussed above [55-58]. The second problem of estimating the residual life of COPV comes from stress rupture be-havior of high strength fibers. It is well known that silicate glass suffers from static fatigue, i.e., time dependent fracture due to the presence of moisture, aka. stress corrosion cracking. Similar effects exist in COPV made with Kevlar and carbon fibers. Figure 17 shows sus-tained load design curves for three types of COPV. These are used for determining the allow-able sustained-load operating stress for a specified time at load using a probability of survival of 0.999. The time at pressure represents the sum of the time that the COPV is pressurized at or above 60% of mean expected operating pressure [81]. Over a long period of service, al-lowable applied stresses are quite low relative to the static strength. Of the three fibers, car-bon fibers give the best performance at the longest time (>100 hrs).

Figure 17 Sustained load design curves for three types of COPV. Data from [81].

Since fiber fracture is the root cause, AE can play a significant role in resolving this problem of estimating the residual life. From the earliest days of AE, the stress rupture issue was rec-ognized and Liptai [5] reported the process of stress rupture of a GFRP NOL ring. See Figure 18a, which follows two hrs of loading before fracture with rising AE counts. This sample was loaded to 80% of the expected fracture load. Another NOL ring was loaded in fatigue at 50% of the ultimate load. At each 2000 cycles, it was loaded to 75% with AE monitoring, shown in Fig. 18b. As listed in the figure, estimated FR values decreased with loading cycles, quick-ly dropping to ~0.5 (as noted earlier in Introduction). Since then, however, only a few reports of AE monitoring of stress rupture of FRP appeared in the last 40 years [81-85]. There are reports of FRP stress rupture studies [e.g., 86-88]. NASA conducted wide-ranging study of Kevlar COPV used in Space Shuttles (24 used in each orbiter vehicle) after the Columbia ac-cident and reestablished the reliability to be 0.998 relying mainly on accelerated pressuriza-tion testing. Raman spectroscopy was used to evaluate the residual stress state of Kevlar [89]. No mention of AE appeared in the report. However, another program is in progress [23,55-58,85] at NASA. On stress rupture tests, only a glimpse of data can be viewed so far [90],

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a) b) Figure 18 a) Cummulative AE counts vs. time at 80% of fracture load applied. b) AE counts vs. load under fa-

tigue load with intermittent proofing. Data from [5]. showing the usual increase in AE just prior to failure. In some cases, other data appear incon-sistent [85] and we need to wait for the completion of the program. Still the location data on a successful test of a COPV shows several concentrations of energetic AE events within 24 hrs of final failure [slide 19, 90]. Two views are given in Figure 19. Such data point to the avail-ability of information that warns incipient failure if continuous AE monitoring is used. With-out monitoring, projecting 0.999-level reliability will probably require a new strategy yet to be formulated.

Figure 19 Distribution of energetic AE events in 24-hrs period before the final fracture. Kevlar COPV. Data from [90] with color inverted for easier viewing.

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Pressure vessels and tanks 2 AE tests of smaller tanks have occupied some European AE groups of late. These have to be done at low cost and rapidly. See [91,92a,b]. A similar effort was undertaken by Propane Ed-ucation & Research Council in the US, using ISO11119-3 as the basis. They chose to use on-ly visual inspection, relying instead on the safety factor [93]. They stated in their Exemption request to the US Dept. of Transportation (prepared by Battelle) that Electronic methods which have been examined to some level of detail in the laboratory include acoustic emission and acousto-ultrasonics (sic). Other potential inspection methods include holography, shearography, and thermography. These methods are used by highly trained staff in controlled laboratory and manufacturing plant conditions for quality control and special purpose applications. However, available data indicate the quality of damage detection in composite cylinders by these methods is inconsistent and varies significantly with the material selection, design, and man-ufacturing techniques. Consequently, unique NDI test criteria will be necessary for each different cylinder de-sign, even though they may be from the same manufacturer. Practical application of these methods will be errat-ic and inconsistent for operations which inspect more than one cylinder type at one time, as is typically done in cylinder and propane industries. Their products, LPG/CNG cylinders, are inexpensive, requiring the lowest cost inspection method. Yet, this seems to be the state of general NDT community that is so ignorant of the state-of-the-art in AE technology. The safety factors they incorpo-rated are still of interest; This value, minimum stress ratio, is defined as the ratio of the fiber stress at calcu-lated design minimum burst pressure divided by the fiber stress at 2/3 times test pressure (i.e. working pressure). For cylinders without liners or with non-load-sharing liners, the stress ratios are:

Glass fibers 3.4 Aramid fibers 3.1 Carbon fibers 2.4

These cylinders are designed to last 15 to 50 years and the minimum stress ratio is to assure against failure due to stress rupture. Note CF can be stressed the highest.